1. Field of the Invention
This invention relates to an electric furnace, and more particularly to an improvement of an electric furnace.
2. Description of the Prior Art
In the electric furnace, generally, entire inner surface of the electric furnace is covered with a heating member and between the furnace cover and the outer surface of the heating member a heat insulating material is inserted in order that the temperature of the heating member will be as close as possible to the inside temperature of the furnace so that the uniform heating property can be increased.
However, such an electric furnace is hardly used because of problems, such as change in configuration due to the heat expansion and increase of cost due to the setting of electric resistors or current paths according to the electric capacity.
In the conventional electric furnace, therefore, a serpentine heating wire or heating coil is wound around the inner surface of the electric furnace and supported by the heat insulating member, electric current is supplied to the heating wire causing it to radiate. In cases where the electric furnace is used for heating it is very important to maintain high temperature, uniform heating and elevating and lowering temperature properties. If the heating wire is of large diameter it is formed helically as it is.
In the electric furnace using the serpentine heating wire it is necessary to make the heating wire with a fine pitch in order to obtain a sufficient effective area facing the center of the furnace, and thus the number of manufacturing steps becomes large and the wire configuration is changed easily by thermal expansion. Accordingly, the heating wire must be adhered to the inner surface of the heat insulating material by using a heat insulating coating material. However, as the heating wire is changed in shape due to heat expansion and because of the fall off of the coating material at the high temperature atmosphere the wire is subject to being easily broken.
In the electric furnace using the heating wire of large diameter, if the effective area facing the center of the furnace is large the percentage of the cost of materials in the electric furnace becomes very large near to 50% and the heat capacity of the heating wire itself is large, so that it is difficult to obtain a good elevating and lowering temperature property. Further, a sag is generated easily in the heating wire at a high temperature atmosphere, so that the heating wire is liable to short or break down.